Primary button should default to 'Save' for users without 'publish_posts'

Description

Synopsis: For users without the 'publish_posts' capability, the primary action for a post should be 'Save' instead of 'Publish'.

Contributor is the most common user role without the 'publish_posts' capability. The current user experience for someone using the iOS app is partially broken because, after creating a post, their call to action is to publish the post, which they don't have rights to do. If they click 'Publish', the XML-RPC call will be made and they'll receive an error that they can't publish posts.

Similarly, the 'Publish' status should be removed from available post statuses if the user doesn't have the 'publish_posts' capability.